Update `babel-preset-es2015` to `v6.6.0`
Update `babel-preset-es2015` to `v6.6.0`

Close h5bp/html5-boilerplate#1797

rp528/html5-boilerplate,vkhorozian/fivePage,ebramk/IS218,snehaltandel/newboilerplate,geosu93/HelloWorld1,gagiksargsyan/html5-boilerplate,AllisonM114/Final-Project,larmour/html-files,byog/html5-boilerplate,MeilinLu/html5-boilerplate,rp528/html5-boilerplate,Deepakzz/assignment1,wvspeters/boilandtrouble,Krisrinidhi/html5-boilerplate,sap64/mynewrepo,bchok/boilerplate4,sjp46/assignment3,katy-400/Boiler,SabaNabeel/HTML-Boilerplate,RanesJan/is218,BlazerR/Assignment1,janina2489/as1,h5bp/html5-boilerplate,AllisonM114/Final-Project,SabaNabeel/HTML-Boilerplate,jeffreysantos/is218,hpaghdal/assignment3,gagiksargsyan/html5-boilerplate-less,diehard226/is117html5,bchok/boilerplate4,domingle/my-website,taupecat/career-day,pankaj-tulaskar/wsda1,AllisonM114/IS-218-Homework1,soumitad/html5_boilerplate,pba3/htmlboilerplate,ars66/boilerplate,sumathi89/html5-boilerplate,janina2489/as1,hpaghdal/assignment3,katy-400/Boiler,larmour/html-files,RanikaR/Assignment_1,JohnnyWalkerDesign/html5-boilerplate,roblarsen/html5-boilerplate,AllisonM114/Is117GitWebsite,ars66/boilerplate,Krisrinidhi/html5-boilerplate,AllisonM114/Is117GitWebsite,byog/html5-boilerplate,swarrier16/boilerCommits,ryandnieves/project1,MaitreyPatel/assigment1,Upmacis-Andrejs/1st_demo_project_for_SEM.lv_Front-end,diehard226/is117website,sumitrawat1/assignment1,rprouse/html5-boilerplate,Deepakzz/assignment1,rprouse/html5-boilerplate,diehard226/is117website,sg772/Assignment1,beera421/myfirstrepo,BlazerR/Assignment1,wvspeters/boilandtrouble,swarrier16/boilerCommits,parepallysvp/clone1,Mounika15/html5-boilerplate,ankurgyani/html5boilerplate,diehard226/is117finalproject,larmour/html-files,Upmacis-Andrejs/untrips_Front-end,Terminocity/terminocity.github.io,MaitreyPatel/assigment1,nn265/niceproject,ryandnieves/improve,deftcat/Fei_Boilerplate,sumitrawat1/assignment1,JohnnyWalkerDesign/html5-boilerplate,AllisonM114/IS-218-Homework1,snehaltandel/newboilerplate,diehard226/is117html5,pankaj-tulaskar/wsda1,nn265/niceproject,gagiksargsyan/html5-boilerplate,RanesJan/is218,sap64/mynewrepo,Kevaughn/Assignment1,balakumarhs/Balakumar,Upmacis-Andrejs/12guru_Front-end,parepallysvp/clone1,gast499/IS218_assignment1,beera421/myfirstrepo,RanesJan/is218,gast499/IS218_assignment1,mtf22/Assignment-1,soumitad/html5_boilerplate,rb496/html5-boilerplate,MeilinLu/html5-boilerplate,vkhorozian/fivePage,balakumarhs/Balakumar,Upmacis-Andrejs/2nd_demo_project_for_SEM.lv_Front-end,h5bp/html5-boilerplate,jeffreysantos/is218,ryandnieves/improve,ebramk/IS218,Upmacis-Andrejs/1st_demo_project_for_SEM.lv_Front-end,deftcat/Fei_Boilerplate,Gia636/clone,fizzvr/html5-boilerplate,fizzvr/html5-boilerplate,sumathi89/html5-boilerplate,domingle/my-website,ronakt307/html5boilerplate,StalynArroyo/Assignment1,sjp46/timeline,majaradichevich/majaradichevich.github.io,geosu93/HelloWorld1,ankurgyani/html5boilerplate,mtf22/Assignment-1,gagiksargsyan/html5-boilerplate-less,pba3/htmlboilerplate,taupecat/career-day,sjp46/assignment3,RanikaR/Assignment_1,diehard226/is117finalproject,MourviTrivedi/boilerplate,StalynArroyo/Assignment1,Terminocity/terminocity.github.io,sjp46/timeline,ronakt307/html5boilerplate,Upmacis-Andrejs/untrips_Front-end,Upmacis-Andrejs/12guru_Front-end,MourviTrivedi/boilerplate,roblarsen/html5-boilerplate,Upmacis-Andrejs/2nd_demo_project_for_SEM.lv_Front-end,majaradichevich/majaradichevich.github.io,sg772/Assignment1,ryandnieves/project1,Mounika15/html5-boilerplate,rb496/html5-boilerplate,Kevaughn/Assignment1,Gia636/clone